fix bug: unitialized constant NilClassDecorator no need to decorate an empty array anyways

[aokolish]

I'm using draper on a project that uses ActiveResource and ran into this problem.

• would like to use implicit version of decorates_assocation
• this works fine because of this line https://github.com/drapergem/draper/blob/master/lib/draper/decorator.rb#L74
• but, if the association returns an empty array, it blows up

For example:

#given, Product has_many :attributes
# without draper involved
product.attributes 
# => []

# with draper, still no attributes
product = ProductDecorator.find(params[:id])
product.attributes
# => uninitialized constant NilClassDecorator

I think it would be better if draper just returned the empty array in this case.

[steveklabnik]

Agreed. Thank you!

[steveklabnik]

(Oh, and just so you know, we pulled ActiveResource out of Rails, so it's a gem now. It'll be maintained by rails core until rails 5)

[aokolish]

Yup, I know all about that. Thanks for accepting this patch!